Beth - I am glad that you know where you stand on this decision already!
I can see the practical need for you to decide how best to let your friend know your decision not to use Sonlight - in a way that is honest yet hopefully does not offend her.
Maybe you can borrow some of my reasons I would share with my friend if I was in your situation...
*We've always wanted to homeschool within our own home, with just our own family.
*I am homeschooling because I want to be my dc's teacher.
*I want our own dc to be each other's best friends, and being home together encourages that.
*I need special 1:1 time every day to share my faith with my dc, and while other moms may get that done in some other way, I know I have to have time every day to be able to do this in a personal way.
*When my dh calls home during the day from work, each of the dc love to talk with him. They need to be home to be able to do that.
*We like to be in control of what we do each day, and having a certain place we have to be each day for a certain amount of time is something we don't want to be committed to do.
*We like the Charlotte Mason method of choosing living books and savoring them, which means we'll spend more time in fewer books rather than reading so many.
*We want Bible integrated in every subject, rather than as a stand alone subject.
*We want our dc to get out their Bibles and use them every single day within our homeschooling.
*We like the short lesson Charlotte Mason style of learning, with bite-sized readings and skill-based follow-ups.
*We want our dc to take over the bulk of their own reading when they turn 9 or 10 yo, so they can retain what they've read better, and so we can spend time in deeper discussions, more connections, and learning new skills, rather than so much time reading.
*We want fine arts integrated in our daily learning, such as music, poetry, art study, etc.
*We want the LA skills of copywork, oral narration, and written narration integrated with history and science as a part of our daily learning.
*We want the organization of the Student Notebook, as it teaches my dc much about organizing their work.
*We want our dc to learn to put together their own portfolio each year.
*We like the moral discernment for book selections and Godly character trait discussions taught in "Drawn into the Heart of Reading", as well as the reading skills focus.
*We want our dc to complete the step-by-step projects in the History Project box as they teach valuable life skills too, such as cooking, project development, and direction following.
*We want our dc to become the scientists themselves by doing their own experiment every single week. I know I would not get the science experiments done otherwise. We also like having them do their science in our own kitchen and our own home, as it often results in them repeating the experiments or taking them further later in the day or week.
*We want to use living books for science.
*We want to customize our school choices for each of our dc, and make sure we are not reading too mature content too young for them.
*We want our dc to learn to do their own Bible Quiet Time first thing in the morning, and HOD has that as part of their plans.
*I like variety in what I teach each year, and HOD changes my teaching time each year.
*I want to choose the proper placement for each of my dc, rather than having them placed together in whatever core is being offered.
*We want our dc to learn to use media resources for research, such as the Internet, audio CD's, or additional books, but we want to be sitting with them as they do this to be sure we are monitoring what they are viewing/reading carefully.
*We want daily hands-on activities that are short, doable, and fun.
*We want to finish school in a timely fashion so we have time for the dc to explore other interests.
I hope something here helps, this is just off the top of my head. Most of these reasons are for you - for you to have in mind so you know why you chose what you chose. They don't all need to be shared with your friend, probably just a few will do. Probably the ones I listed first would be best to share with her, as they have more to do with the schooling being done out of the home than with "Sonlight". Sonlight and HOD differences will be a "hot button", I wouldn't push it unless you have to.
Vague is sometimes better. If she is happy with Sonlight, no reason to rock her happy boat.
No reason for you to jump in her boat either though.
I have found that I have to be alright with my decisions, and know in my soul they are right for our family. I have learned to be able to stand alone sometimes because of this. There are people who are my friends and even who are part of my extended family that don't agree with my decisions. They have not been convinced that what we are doing is right. I do not try to convince them or win them over. I have found time does that for me. As my dc are growing and maturing, there is something special about them. I'm not trying to be prideful here, but they just are different. They are innocent. They have an easy rapport with each other, with my dh and me, and socially with adults and dc alike. They know their academics. They know their Lord. It shines through, and slowly but surely, others are having a grudging respect for that. They are not perfect, but they are not as much "of this world" as I was at their age. I love that. The Lord must too. I am praying for strength for you as you discuss this with your friend. I hope something here has helped!
Love in Christ,
Julie